Transcript Lasers and the Space Elevator

Lasers and the Space
Elevator
Adaleena Mookerjee
August 16, 2006
Center for Structures in Extreme
Environments, Rutgers University
What are Lasers?
LASER (light amplification by stimulated
emission of radiation)
A device that controls the way energized atoms
release photons
Laser History
Conceptually
developed by Albert
Einstein in 1917
Actually built by
Theodore Maiman in
the 1960s (Ruby laser)
Previously, was not
important at all
Today, it is all around
us from printers to
barcode readers at the
bookstore or mall
Basic Phenomena of Light
Wave duality of matter: light is capable of
behaving like a wave and particle
Light – The Wave
Any electromagnetic
radiation with a
wavelength visible to the
eye
3 properties of light:
– Intensity (amplitude) –
how bright something
appears to the human
eye
– Frequency (wavelength)
– color of light produced
– Polarization – angle of
vibration of light
Light the Photon
protons
Photons – quanta of
electromagnetic radiation
which can be light
Bohr Model:
– Made up of 3 subatomic
particles
Protons (positively charged
subatomic particle)
Neutrons (neutrally charged
subatomic particle)
Electrons (negatively charged
subatomic particle)
– Protons and neutrons are
located in nucleus
– Electrons are located in a
hypothetical region
surrounding the nucleus
electrons
neutrons
Producing Light
Step 1: Light is emitted and
photons are released,
colliding with the orbiting
electron
Step 2: Collision causes change
in velocity and position,
making the electron absorb
photon’s energy
Step 3: Electron moves to a
higher energy level or
position around nucleus
(excited!)
Step 4: To return, electron
releases energy from photon,
producing light
c  f
Wave and Photon Correlation
Speed of light (c) = 300,000 km/s
Relationship between photon and
wave:
c  f
Where λ = wavelength and f = frequency
Terminology – Photons
Population inversion: when a
system of either a group of
molecules or molecules exist in a
state where more electrons are in
an excited state than in the lowest
energy level possible
Absorption: photon with a
particular frequency hits an atom
at rest & excites it to a higher
energy level while photon is
absorbed
Spontaneous emission: atom in an
excited state emits a photon with a
particular frequency & returns to
ground state
Stimulated emission: photon with
a certain frequency hits excited
atom & releases two photons of
same frequency while electron
returns to ground state
http://perg.phys.ksu.edu/vqm/laserweb/index
.html
Terminology -- Waves
Scattering: when atoms of a
transparent material is not
smoothly distributed over
distances greater than the
length of a light wave
Reflection: light normally
collides with the boundary
of 2 materials
– Objects contain free
electrons which jump
from one atom to
another within it
– Energized electrons
vibrate  sends back of
object as light wave with
same frequency of
incoming wave
– Does not deeply pierce
material
Refraction: bending of
light when it passes from
one kind of material to
another
– Frequency of incoming
light matches natural
frequency in electrons
– Penetrates deeply into
material causing
vibrations in electrons
– Waves slow down and
outside it maintains
original frequency
Examples
scattering
reflection
refraction
Goals of Presentation
Discuss what exactly a
laser is
Discuss how a laser
works and how to build
your own
Discuss the types of
lasers available today
(solid, gas, liquid,
semiconductor,
excimer, free electron)
Propose the best laser
for the space elevator
What is a Laser? (in words)
Did you know? Actually, it is an acronym: LASER (light
amplification by stimulated emission of radiation)
A device that controls the way energized atoms release
photons
Laser light is very intense, highly directional & pure in
color
Not very safe to look directly at laser light
4 types of lasers:
1) solid state lasers
**Other forms also are
2) gas lasers
excimer and free electron
lasers, but they don’t fall
3) liquid lasers
into any of these
4) semiconductor lasers
categories.
Classified based on gain medium (that will be
defined in one slide…) used
Parts of a Laser
Energy source:
--begins the lasing process
--examples include: electrical
discharges, flashlamps,
arclamps, lights from another
laser, lights from chemical
reactions, lights from
explosions
Output Coupler:
--where light is allowed to come
out
--semitransparent mirrors
--controls effective output of
light produced
Gain Medium (excitation
mechanism):
--transfers external energy
to beam
--excites particles
--keeps laser at desired
wavelength
--absorbs energy in the
laser
--maintains the laser
Optical Resonator
(feedback): arrangement
of optical components
allowing beam to circulate
Laser Function & Construction
As you noticed, light is
commonly used as the
energy source and the
gain medium
Energy source applied to
system
Gain medium transfers
energy to beam,
energizing electrons
Electrons give off light
energy to return back to
its original energy level
Resonators produce
more laser light
Need to have knowledge
in glassblowing,
fabricating small parts,
operating a vacuum
Use a solid, liquid or gas
medium (best gas is
nitrogen)
Two resonating mirrors
are used to reflect light
formed
Energy source applied to
the system
Emission of photons will
result in light
Solid-State Lasers
Uses a gain medium
which is a solid (not
semiconductor)
Energy source: flashlamp
or arclamp
Optical resonator: two
mirrors in parallel
Produces power ranging
from milliwatts to
kilowatts  light lasts for
short durations
The Ruby Laser (a solid state
laser)
1st laser by Theodore
Maiman
Used a synthetic ruby and
made it in shape of cylinder
Wrapped it around a high
intensity lamp
The blue and green
wavelengths from the white
light triggered an
excitement within electrons
of chromium atom
When returned to stable
state, they released energy
in form of ruby light
Phenomena continued until
critical level reached and
pulse released
Implementation on the Space
Elevator
High powered solid state
lasers may be successful
with providing power
Obtaining solids may be a
problem
Most acquirable item
today is Nd:YAG
(neodymium doped
yttrium aluminum garnet)
Produces very limited
power
Gas Lasers
Active Medium: pure gas,
mixture of gases, metal
vapor
Energy Source: electrical
discharge, flashlamp,
arclamp, light from other
laser, chemical reaction
or explosion
Optical Resonator: 2
mirrors in parallel to each
other
Power Generated: 50
watts to 4 kilowatts of
power
CO2, N2 and HeCd Lasers
CO2 Lasers:
HeCd Lasers:
--uses CO2 to begin lasing
process
--metal is cooked
and vaporized
--Active medium: 1 carbon
dioxide, 1 nitrogen gas, 1
helium
--helium excited
by collisions with
excited electrons
--high voltage power
added to system
--Process:
--pass on to
cadmium atoms
--creates electrical
discharge & population
inversion
1) nitrogen added,
exciting carbon dioxide
2) helium added to remove
electrons from lowest
energy level (population
inversion)
3) tube sealed and voltage
added exciting system
--cadmium
heated and
added to helium
gas
--helium fills
cavity while
cadmium goes to
cathode
N2 Lasers:
--uses N2 as active
medium
--laser acts for short
time
--good for scientific
research, pumping other
lasers
--minimal damages
Implementation on the Space
Elevator
Possibly a good idea
Could utilize gases
available on Earth in
excess, reducing
pollution or its harmful
effects
Negative effect: too
much gas or some
gases could corrode
elevator, cause
possible explosions and
potentially damage the
Earth
Moderate amounts of
gas needed for use
Liquid Lasers
Active Medium: liquid
Energy Source: light from
another laser
Optical Resonator: 2
mirrors in parallel with
one another
Power Generated: few
watts covering radius of
20 micrometers
Tunable over a wide
range and produces a
broad range of colors in
visible spectrum
Dye Lasers
Uses organic liquid
dyes as active
medium
1 cm long quartz
glass tube
Dye cell: inside of the
tube which consists of
partially reflective
mirrors on the front &
diffraction grating on
rear
Laser Action:
– Energy added by a light
source such as
flashlamp/laser
– Dye absorbs wavelength of
light shorter than what it
emits and input energy in
forms of energy & heat
– Absorbed energy creates a
population inversion
(electrons excited)
– Vibrational energy loss
causes dye molecules to
go into lowest energy state
– Emission occurs when
vibrational levels reach
ground state.
Implementation on the Space
Elevator
Not practical
Provides too little power
Not feasible to use organic
dye (harder to maintain)
Dye could get stale over time
(requires replacement every
few days)
Could cause malfunction of
the laser if dye is not
replaced
Dependent on another laser
for starting, so two lasers
would be necessary
Semiconductor Lasers
Active Medium:
semiconductor solid
(solid which conducts
electricity) – needs to
confine carrier & take
up small volume
Energy Source:
electrical impulse
1st semiconductor:
1962
– Coherent
electromagnetic
radiation produced
by a p-n junction
using GaAs
Semiconductor Terminology
p-type semiconductor:
semiconductor in which
electrical conduction is
due chiefly to the
movement of positive
holes
n-type semiconductor:
electrical conduction due
chiefly to the movement
of electrons
p-n junction: where p-type
semiconductor is
adjoined with the n-type
semiconductor
Valence band: where
highest energy level has full
number of electrons
Conduction band: where
lowest energy level has no
electrons
Band gap (energy gap):
space between valence
band and conduction band
Minority carrier: contains
few mobile electrons in ptype semiconductor region
& free holes in n-type
semiconductor region
Majority carrier: free holes
in p-type region & electrons
in n-type region
Semiconductor Lasing
Population Inversion: charge
carriers (electrons) cross p-n
junctions  minority carriers
Minority carriers mix with
majority carriers
Photon is absorbed by
electrons (gives energy to
jump from valence to
conduction)
Leads to stimulated emission,
releasing a photon
Optical resonator reflects the
light out & sometimes back
into the solid
Implementation on the Space
Elevator
Power Generated:
few milliwatts
Will not produce
enough power
Small in size & won’t
create enough light
Option: use a system
of semiconductor
lasers  pretty costly
Excimer Lasers
Active Medium: noble gas
(argon, krypton, xenon) +
halogen (fluorine,
chlorine, bromine, iodine)
– Exists for 10 nanoseconds
during excited state
– In ground state, exists as
separate atoms
Energy Source: UV light
Optical Resonator: 2
mirrors in parallel to each
other
Excimer Lasers
Chemical Composition:
– 0.1-0.2% halogen
– Little noble gas
– 90% of helium or neon
Laser Action:
– When electrical discharge or energy is added to noble
gas, can bind to halogen (excited)
– Gives up additional energy through stimulated
emission, forming ground state molecule
– Within picoseconds, can separate into 2 atoms 
population inversion
Implementation on the Space
Elevator
FACT: sun is composed
of hydrogen, helium,
oxygen, carbon, iron,
neon, nitrogen, silicon,
magnesium & sulfur
Use solar radiation as
energy source for the
laser (excess sunlight)
Lasing process is for few
nanoseconds, but power
generated: few watts to
few hundreds of watts
Radiation exposure will
be minimal, but effective
for the space elevator
Free Electron Lasers
Best laser according to
Edwards & Westling
Device which emits high
powered electromagnetic
radiation at any
wavelength
Contains an array of
magnets in magnetic field
to excite free unbound
electrons
Tunable over broad range
of wavelengths
Class IV lasers: capable
of starting fires, burn flesh
and cause eye damage
Free Electron Lasers
Beam of electrons
accelerated to relativistic
speeds (electron
accelerator)
Electrons pass through
periodic, transverse
magnetic field
Magnetic field causes
electrons to travel at a
sinusoidal path
Electrons move at higher
speeds, releasing
photons
Optical mirrors lengthen
process
Implementation on the Space
Elevator
Efficiency of 65%
Can emit radiation at
any wavelength
(tunable)
Accelerated electrons
release x-rays at
hazardous levels
Produces high
quantities of power
Electron accelerator is
big
Very expensive
Conclusions
Best Laser: gas or excimer laser
Why? – can use gases in the solar system or
atmosphere
Other lasers would require further research
Other Factors to Study:
– Threshold of maximum gas needed
– Overall harmful effects of lasers for necessary
protection
– Some type of radiation shielding
– What should be done if gas runs out?
– Process of the laser
– Strength to endure effects of nature
Acknowledgements
At this time, I would like to thank:
Professor Benaroya for giving me this
opportunity to learn about lasers
Yuriy Gulak for setting me up here and
familiarizing me with the technology
available here
Everyone of you for teaching me about
your research & making me feel
comfortable here